Spatial patterns of stable isotopes in precipitation in Switzerland for the use in hydrological and hydrogeological applications

The stable isotopes of the water molecule, oxygen-18 and deuterium, provide an ideal tracer for water movement and offer a broad range of possibilities to study different processes in the water cycle. To be used in hydrological analyses and modelling the isotope data in precipitation as an “input” function has to be known for the particular catchment or point of interest. This paper presents the development and application of a method interpolating – on a monthly basis – stable isotope data in precipitation of the isotope observation network in Switzerland (ISOT), a module of the NAQUA National Groundwater Monitoring. Several influencing variables (e.g., topographical parameters, climate variables) are tested in a multi-regression framework, and the residuals are interpolated by the use of ordinary kriging. The different variants are tested by cross-validation, splitting the sample in two periods. The tests also provide information about regional differences of the interpolation quality in Switzerland, from which recommendations are made to densify the existing network. Maps of oxygen-18 and deuterium in a 500 m raster are delivered for selected months and years. As a further step in this study, for particular measurement sites of groundwater and surface water with known catchments, the “input” function is determined and compared to the measurements to (i) further validate the interpolation method and, (ii) to improve existing hydrological and hydrogeological information about the location of the recharge area and mean travel times. Also, the “input” functions can be used in hydrological modelling of combined water movement and solute transport in water quality studies.